Thoreau’s 1849 land Survey Quarter Acre Miscalculation

Thoreau worked as a professional surveyor, creating over 100 land and property surveys from 1846-1860.

The Concord free Public Library (CFPL) curates and provides on-line public access to digital copies of Thoreau’s original “Land and Property Surveys” and microfilm public access to images of Thoreau’s original“Field Notes of Surveys Recorded by Henry D. Thoreau (November 1849 – December 1860)” .

Thoreau’s earliest surveys on file at the CFPL are his 1846 surveys of Walden Pond’s subsurface topography.

The outer perimeter consists of eleven points labeled A to L, (no J), that define a series of eleven two point edges. The length of each edge was measured using a surveyor’s chain and recorded in units of rods and links. The compass orientation for each edge also is shown.

In order to compute the area of Isaac Watt’s Woodlot, Thoreau identified and labeled 38 rectangular quarter acre lots within its perimeter plus 14 odd shaped lots around its periphery.

The east corner of the survey image (lower-right, above) includes a column of values for the 14 odd shaped lots and a total value. Thoreau made a math error when summing the 14 values.

The 14 odd shaped lots on the periphery listed and numbered by Thoreau include:

92 square rods in lot # 15 (lower right)

58 square rods in lot # 01 (lower left)

70 square rods in lot # 33 (middle left)

65 square rods in lot # 18 (above lower right)

60 square rods in lot # 19+ (above #18)

60 square rods in lot # 44+ (above #19)

70 square rods in lot # 45 (upper right)

60 square rods in lot # 52 (above #45)

140 square rods in lot # 51 (left of #52

30 square rods in lot # 50 (below #51)

50 square rods in lot # 37 (below left of #50)

62 square rods in lot # 36 (left of #37)

64 square rods in lot # 35 (left of #36)

66square rods in lot # 34 (left of #35)

Thoreau incorrectly computed the total area for those lots as 907 square rods = 5.66 acres. (160 square rods = one acre)

The correct sum of the data for the 14 odd shaped lots is 947square rods = 5.91 acres, a difference of 40 square rods or a 0.25-acre under count by Thoreau.

At the end of his“Field Notes of Surveys” for Isaac Watt’s WoodlotThoreau said: “Surveyed 52 lots of a quarter of an acre and some cases more.”

All but one of the odd shaped lots contains more than a quarter acre.

In summary,Thoreau undercounted the total acreage of Isaac Watt’s Woodlot by 0.25 acres on his worksheet, but his survey map is correct in every respect except for his note summarizing the area of the odd shaped lots.

Thoreau said he surveyed 52 lots which is correct and the average lot size for the combined quarter acre 38 uniform shaped lots and 14 odd shaped lots equals slightly more than 15 acres, just as Thoreau described “52 lots of a quarter of an acre and some cases more”.

So although Thoreau’s list of odd shaped lots appears to contain an under count of 40 rods or 0.25 acre, the error as a percent of the total area is small (0.25/15= 1.7%)

Thoreau mentions other known inaccuracies such as “Greatest error in IK & KL” which appears to be a zone edge approximation along its Virginia Road boundary across the road from Thoreau’s birthplace and current Headquarters of the Thoreau Society.

Survey # 135 can be difficult to locate on the ground today but I believe its northeast corner is at the location of Thoreau’s birthplace on Virginia Road that is also the current Headquarters of the Thoreau Society.

See below:

The survey’s western most corner that Thoreau refers to as point “A” on his sketch is at the former Headquarters of the Thoreau Society on Old Bedford Road. (Approx.)

Moss states that: “I think it was the woods back of the old Kettell place on Lexington Road. It was divided into 52 lots and cut in 1849-50.”1

It is not known whether Thoreau ever described the total acreage surveyed as 15.56or 15.41 acres other than his notation “907/160=5.6” which should have been 947/160=5.9. Thoreau’s survey drawing is accurate in locating and labeling each of the 52 lots.

Thoreau’s math error was a minor error because the final survey document is graphically correct in every respect. Every one of the 52 woodlots is correctly described and only his intermediate summation of the area of 13 irregular lots appears to have an under count error by a quarter acre.

As he stated, Thoreau had recorded a survey of “52 lots of a quarter of an acre and some cases more”for Isaac Watt’s Woodlot during November 1849.

Thoreau’s mapping ability evolved over time as he gained experience, acquired new tools e.g. surveyor’s chains, transits and newly available surveying instructions.

Thoreau’s library included a copy of Elements ofsurveyingand navigation: with a description of the instruments and the necessary tables / by Charles Davies.New York: A.S. Barnes & Co., 1847, c1835. Note: Multiple copies of this book are available online as a result of Google Books copying of several public libraries.

Understanding of Thoreau’s writing for any given time period will benefit from an awareness of Thoreau’s working environment as a surveyor. Thoreau’s References to natural features of his time may be related to the location of Thoreau’s fieldwork as a surveyor and may still be relevant today.

“Isaac Watt’s woodlot” looks like part of “Bedford Levels” on Gleason’s 1906 Map of Concord at the intersection of Virginia Road and Old Bedford Road, a.k.a. “Meriams Corner”.

When locating Isaac Watt’s Woodlot remember its area is one quarter that of Walden Pond that Thoreau reported as 61 acres or 4 times the size of Isaac Watt’s woodlot (15.6/61 = .25.6%) and visible for comparison on Gleason’s 1906 map of Concord.